1. Field of the Invention
The present invention relates to a wavelength router for selectively outputting optical signals from a plurality of input ports to a plurality of output ports.
2. Description of Related Art
Conventionally, known are wavelength routers employing (1) a combination of a wavelength division multiplexing and de-multiplexing element and a light switch, (2) a combination of an array waveguide diffraction grating and a phase element, and (3) a variable optical fiber grating. These wavelength router devices can be classified broadly into a type which switches the path of signals per wavelength after having separated every wavelength division multiplexing signal into signals of respective wavelengths (includes aforementioned (1) and (2)), and a type which switches the path only for signals with a specific wavelength (includes aforementioned (3)). Pursuant to the increase in traffic capacity, it is preferable to use the wavelength router device of the latter type as the number of wavelength channels is increasing rapidly at a rate of exceeding 1000 waves.
The latter type of wavelength router devices includes, for example, a wavelength router device employing the variable optical fiber grating. With this device, the number of signals in which the path thereof may be switched; that is, the number of wavelengths, coincides with the number of elements (variable optical fiber gratings). Moreover, there is a wavelength router device employing the acoustooptical (AO) filter element. With this device, it is possible to control a plurality of wavelengths with a single acoustooptical filter element.
A wavelength router device employing the acoustooptical filter element is disclosed in the document, xe2x80x9cShingaku Giho, PS99-68, pp. 31-35, January 2000xe2x80x9d.
The structure and method of using a typical acoustooptical filter element is now explained with reference to the plan view of FIG. 6. The acoustooptical filter element 10 comprises two input ports 12a and 12b, and two output ports 14a and 14b. For instance, the path from the input port 12a to the output port 14a is used as the trunk line. Here, the input port 12b is used as the optical signal insertion port and the output port 14b is used as the optical signal removal port. Such usage is referred to as OADM (Optical Add Drop Multiplexing).
The acoustooptical filter 10 is also structured by employing a substrate 16 having an acoustooptical effect. An optical wave guide is formed between the input port and output port of this substrate 16. Further, polarizers 18 and 20 are respectively formed in the area of the input side and output side of the substrate 16 in the middle of such optical wave guide.
The polarizer 18 on the input side outputs the input light from the input port 12a or 12b after separating it into two mutually perpendicular polarized components. The polarizer 20 on the output side combines the polarized components input thereto, and outputs this to the output port 14a or 14b. Polarized light non-dependency is thus realized.
In addition, a comb-shaped electrode 22 is provided between the polarizers 18 and 20 of the substrate 16 and in a position on the input side close to the polarizer 18. Upon applying a high-frequency voltage to this comb-shaped electrode 22, a surface acoustic wave 24 is excited on the surface of the substrate 16. Among the polarized light propagating between the polarizers 18 and 20, only the light component corresponding to the wavelength of the surface acoustic wave is converted into polarized light. Light converted into polarized light is separated from other light at the polarizer 20 of the output side. By exciting the surface acoustic wave with a plurality of frequencies with the comb-shaped electrode 22, it is possible to select light of a plurality of wavelengths corresponding thereto.
However, acoustooptical filter elements have limited input drive power as the electrodes may be damaged, etc. Therefore, a wavelength router employing acoustooptical filter elements are restricted in the number of wavelengths that it can handle.
Moreover, as acoustooptical filter elements have inferior wavelength resolution, there are problems in that a sub peak may arise next to the main peak in the wavelength spectrum, or beat noise may appear on the main peak if the wavelength approaches even further. Thus, a wavelength router employing acoustooptical filter elements is not capable of accommodating high density wavelength division multiplexing.
Accordingly, a first object of the present invention is to provide a wavelength router suitable for processing signals in wavelength division multiplexing.
A second object of the present invention is to provide a wavelength router capable of reducing the sub peak and reducing the beat noise in the output light.
A third object of the present invention is to provide a wavelength router capable of selecting and outputting a specific signal or a plurality of signals from a plurality of multiplexed wavelength signal groups.
The wavelength router of the present invention comprises a wavelength division multiplicity reduction element and an acoustooptical filter element. The wavelength division multiplicity reduction element has a plurality of output ports and de-multiplexes the input wavelength division multiplexing optical signals and outputs the de-multiplexed optical signals to the output ports, respectively. The acoustooptical filter element is connected to the output ports of the wavelength division multiplicity reduction element, and selectively outputs the optical signals input from the output ports in accordance with the wavelength thereof.
According to this structure, as the multiplexing of the wavelength division multiplexing optical signal is reduced at the stage prior to being input to the acoustooptical filter element, it is suitable for processing high density wavelength division multiplexing signals and allows the reduction of the sub peak in the output optical signals and reduction of the beat noise.